The present invention relates generally to an electrolytic cell assembly made up of a series of bipolar electrodes with diaphragms or membranes sandwiched in between for the production of alkali metal hydroxides and halogens. More particularly the present disclosure relates to an improved bipolar electrode wherein the anode and cathode compartments are pans, each pressed from single sheets of solid metallic materials and assembled in back-to-back spaced relation by suitable electrically conducting means, leaving an air space between the pans. Peripheral channels of the pans are filled with rigidizing material so as to form a solid clamping surface by which to stack the electrodes into a filter press electrolytic cell.
Chlorine and caustic (sodium hydroxide) are essential and large volume commodities which are basic chemicals required in all industrial societies. They are produced almost entirely by electrolysis of aqueous solutions of alkali metal chlorides, with a major proportion of current production coming from the diaphragm type electrolytic cells. These cells have a honeycomb type arrangement of anodes and cathodes with brine (sodium chloride) starting material fed into the cell through the anode compartment. To minimize back-diffusion and migration through the hydraulically permeable diaphragm, the flow rate is always maintained in excess of the conversion rate so that resulting catholyte solution has unchanged alkali metal chloride present. This catholyte solution, containing sodium hydroxide, unchanged sodium chloride, and certain other impurities, must then be concentrated and purified to obtain a marketable sodium hydroxide commodity and a sodium chloride solution to be reused in the diaphragm electrolytic cell. This is a serious drawback since the costs of this concentration and purification process are rising rapidly.
With the advent of technological advances such as the dimensionally stable anode which permits ever narrowing gaps between the electrodes and the hydraulically impermeable membrane, other electrolytic cell structures are being considered. The geometry of the diaphragm cell structure makes it unrealistic to place a planar membrane between the electrodes, hence the filter press electrolytic cell structure has been proposed as an alternate electrolytic cell structure.
A filter press electrolytic cell is a cell consisting of several units in series, as in a filter press, in which each electrode, except the two end electrodes, acts as an anode on one side and a cathode on the other, and the space between these bipolar electrodes is divided into an anode and cathode compartments by a membrane. In a typical operation, alkali metal halide is fed into the anode compartment where halogen gas is generated at the anode. Alkali metal ions are selectively transported through the membrane into the cathode compartment, and combine with hydroxyl ions generated at the cathode by the electrolysis of water to form the alkali metal hydroxides. In this cell the resultant alkali metal hydroxide is sufficiently pure to be commercially marketable, thus eliminating an expensive salt recovery step of processing. Cells where the bipolar electrodes and the diaphragms or membranes are sandwiched into a filter press type construction may be electrically connected in series, with the anode of one connected with the cathode of an adjoining cell through a common structural member or partition. This arrangement is generally known as a bipolar configuration. A bipolar electrode is an electrode without direct metallic connection with the current supply, one face of which acts as an anode and the opposite face as a cathode when an electric current is passed through the cell.
While the bipolar configuration provides a certain economy for electrical connection of these electrodes in series there is a serious problem with the corrosion of cell components in contact with the anolyte. The anolyte normally contains highly corrosive concentrations of free halide, and the use of base metals such as iron to contain the solution have proven to be ineffective.
Proposals to overcome this problem include utilizing valve metals or alloys thereof to contain anolyte, either by fabricating an entire electrode from such a corrosion resistant material or by bonding a coating of valve metal onto a base metal within the anolyte compartment. The use of large quantities of expensive valve metals in commercial cell construction though has proven to be economically impractical. The coated base metals on the other hand are prone to disintegration by pealing off of the protective layer and have also proven ineffective. It would therefore be very advantageous to provide a bipolar electrode wherein corrosion resistant valve metals are used in an economical manner to contain the anolyte, making a filter press electrolytic cell structure a viable commercial alternative for the present diaphragm cell.